Electrochemical oxygen demand system

ABSTRACT

A method and system for the determination of organic and inorganic waste products present in aqueous samples, the arrangement including cell means for receiving a flow of an aqueous sample therethrough with means for delivering a predetermined quantity of a water soluble electrolyte into said sample prior to passing through said cell. Heating means are provided for maintaining the sample at a predetermined temperature level as it passes through said cell. The cell includes a plurality of space electrodes including a reference electrode, an indicating electrode, and a common electrode in circuit with said reference and indicating electrodes. A first circuit is coupled to the reference and common electrodes and a unipolar electrical signal is impressed thereacross, the magnitude being sufficient for aqueous electrolysis, A second circuit means is coupled to said indicating and common electrodes and delivers a scanning signal of a programmed potential magnitude with periodic increases and decreases in respect to time, the peak magnitude of said scanning signal being less than that required for aqueous electrolysis. Readout means are arranged in series with said indicating and common electrodes for determining the current flow through said second circuit means in response to the scanning signal. The system is also useful for the determination of carbon monoxide in gaseous atmospheres.

United States Patent Cummings et al.

[451 July 11, 1972 [54] ELECTROCHEMICAL OXYGEN DEMAND SYSTEM [72]Inventors: John P. Cummings, Minneapolis; Richard E. Berg, Chanhassen,both of Minn.

[51] ..G01n 27/46 [58] Field ofSearch' ..204/1 T, 195 R, 195 B [56]References Cited UNITED STATES PATENTS 2,246,981 6/1941 Matheson et al...204/195 OTHER PUBLICATIONS Kolthoff et al., Polarography, 2nd ed.,1952, pp. 364- 367, 399, 400 & 556

Primary ExaminerT. Tung Attorney-Arth1ir H. Swanson, Lockwood D. Burtonand Mitchell J. Halista [57] ABSTRACT A method and system for thedetermination of organic and inorganic waste products present in aqueoussamples, the arrangement including cell means for receiving a flow of anaqueous sample therethrough with means for delivering a predeterminedquantity of a water soluble electrolyte into said sample prior topassing through said cell. Heating means are provided for maintainingthe sample at a predetermined temperature level as it passes throughsaid cell. The cell includes a plurality of space electrodes including areference electrode, an indicating electrode, and a common electrode incircuit with said reference and indicating electrodes. A first circuitis coupled to the reference and common electrodes and a unipolarelectrical signal is impressed thereacross, the magnitude beingsufficient for aqueous electrolysis, A second circuit means is coupledto said indicating and common electrodes and delivers a scanning signalof a programmed potential magnitude with periodic increases anddecreases in respect to time, the peak magnitude of said scanning signalbeing less than that required for aqueous electrolysis. Readout meansare arranged in series with said indicating and common electrodes fordetemiining the current flow through said second circuit means inresponse to the scanning signal. The system is also useful for thedetermination of carbon monoxide in gaseous atmospheres.

15 Claims, 7 Drawing Figures SCANNER TO REFERENCE ELECTRODE POWER SUPPLYVOLTAGE :Iliffj gill 1:51: Q3211:

SAMPLE 1 17) INPUT PATENTEDJIII I I I972 SHEET 10F 3 SAMPLE CIRCULATINGPUMP 1/5 FLOW CHAMBER OR CELL CURRENT VOLTAGE D.C. SELECTOR SCANNERPOwER SWITCH MECHANICAL SUPPLY I SAMPLE I HEATER 1 2 ACID I REFERENCEHEATER ELECTRODE SUPPLY 25- CONTROL POWER SUPPLY CHAMBER SAMPLETEMPERATURE RECORDER 51 CONTAINER CONTROLLER AND PRE HEATING 52 CHAMBERI Z4 26 SAMPLE J DISCHARGE INCOMING SAMPLE J1 TO VOLTAGE SCANNER TOREFERENCE ELECTRODE POwER SUPPLY 1/8 l L1 j5 L SAMPLE INPUT 15 INVENTORSday/u P Cl/MM/A/GS BY IP/CHAED 5 652a PATE'N'IEDJIII I I I972 CURRENT(MICROAMPS) CURRENT (MICROAMPS) SHEET 2 OF 3 TANKs RELATIVE AMOUNT OFI60 UNSTABILIZED QK ALGAE EFFLUENT SEWAGE 80 PLANT I TANK 3 4o RELATIVECONCENTRATION 0- OF DISSOLVED RELATIVE OXYGEN CONCENTRATION OF DISSOLVEDCHLORIDE IoN I I l I I I l I I 4| 0 I .2 3 4 .5 .6 .7 .8 .9 IO U L2 L3L4 L5 I6 I? L8 L9 APPLIED POTENTIAL (VOLTS) No DIFFERENCE DISSOLVED INCHLORIDE OXYGEN LEVEL CoNCENTRATIoNs Low IN CREEK CREEK SURFACE 8oSAMPLE UNSTABILIZED ORGANICS HIGH 4o IN CREEK LAKE SURFACE SAMPLE L I II I I I I l I I I I O l .2 .3 .4 .5 .6 .7 .8 .9 IO M 1.2 L3 L4 .5 L6 L7L8 L9 INVENTORS APPLIED POTENTIAL (VOLTS) dOH/I/ P CUMM/A/GS 5 BISQG-ATTOQ/UEP FIE 1 PATENTEDJUL 1 1 I972 SHEET 30F 3 r v ajs 45 MOTOR .15 AC41 PUMP I 42 BI 1 39011 To I "I REFERENCE 47 4g ELECTRODE I 5 TO 50 senINDICATOR L ELECTRODE TO REFERENCE ELECTRODE l7 COMPRESSOR REFERENCEELECTRODE l5 \IIB EXHAUST 6f CONNECTION RECORDER REA DOUT TO SOURCEOFCARBON MONOXIDE RECORDER JACK I 68 REFERENCE L ELECTRODE ELECTRICAL 0cMETER SCAN INPUT T0 POWER READOUT INDICATING CTR s SUPPLY ELECTRODE l6ELE ODE a 2' 3 l0 l4 2 2 g 40 I2 9 i E o o g 6 l3 0 3O 7 3 o l m w 0 5len 0 z 20 4 O a. W m

INVENTORS' o I l l l l l l L PER CENT CARBON MONOXIDE IN GAS SAMPLE FIE7 Q/CHA D E. 0

ATTOE/UEV ELECTROCHEMICAL OXYGEN DEMAND SYSTEM CROSS-REFERENCE TORELATED APPLICATION The present invention is concerned with anelectrolytic technique for determination of organic waste products inaqueous samples, and involves a substantially continuous voltage scannon-exhaustive technique for this determination. A modified system andtechnique for determination of organic waste products in aqueous samplesis disclosed and claimed in the co-pending application of John P.Cummings, a co-inventor herein, entitled Exhaustive Electrolysis SystemFor Determination Of Oxygen Demand, Ser. No. 58,448 executed on evendate herewith, and assigned to the same assignee as the presentinvention.

BACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates to an electrochemical oxygen demand sensor which isutilized to quantitatively determine the organic waste content presentin aqueous samples, including raw, treated, or partially treated sewagesamples, as well as industrial waste products or the like. The inventioninvolves an electrolytic determination wherein a predetermined quantityor concentration of an electrolyte is provided for a sample and ascanning voltage is applied across a pair of electrodes disposed in thesample, and diffusion and subsequent reaction of the species undergoingdetermination provides an output reading from an indicating electrodewhich is indicative of certain solution parameters including thequantity of dissolved oxygen present, as well as unstabilized organicsand chloride ion concentrations. Since the parameter being measured isessentially the electro-activity, the system may be utilized for carbonmonoxide detection.

In addition to the determination of water quality, or the determinationof sewage treatment effectiveness, it is frequently desirable to locatesources of pollution influx into a body of water, such as a natural orartificial lake. These sources of pollution may occur from incomingstreams, private sewage disposal fields for residences or dwellings, orthe disposal of sewage plane effluents. The dissolved oxygen andchloride ion concentrations may be determined over the area of an entirelake body, and the dissolved organics can be monitored in areas wherethey are concentrated, such as adjacent incoming streams or along thelake bottom. For example, a determination may be made that organics areconcentrating at the bottom of a lake body with the resulting decreasein dissolved oxygen endangering the condition of the lake. Since waterturn-over in a normal natural body of water such as a lake requires aperiod of 10 to 20 years, it is frequently desirable to monitor thewater quality prior to the achieving on an intolerable level ofpollution in the lake.

The dissolved oxygen level in a body or stream of water is alTected bythe action of bacteria upon unstabilized organic waste material presenttherein. Coliform bacteria are present in any human waste stream alongwith nutrients (nitrate and phosphate) for algal growth. The chlorideion concentration is also becoming an important factor because of thetendency for bodies of water to increase in salinity, and accordinglybecome either brackish or possibly corrosive.

2. Description of the Prior Art The oxygen demand of aqueous samples isgenerally regarded as the single most important quality parameterutilized to monitor the quality of a water supply or the efi'ectivenessof treatment of sanitary and certain industrial plant effluents. Theoxygen demand or the amount of dissolved oxygen required for thestabilization of dissolved organic material present in a sample hastraditionally been measured by'means of the 5-day biochemical oxygendemand (BOD) test. Generally speaking, the BOD test determines theoxygen consumed by bacteria during their normal metabolic processes ondissolved and biologically unstabilized organic matter. Under normalconditions of temperature and pressure, about 20 days are required forcomplete stabilization, but practical and reasonably reproducibleresults may be obtained after a period of five days.

For many control purposes, the five-day period is unreasonably long andunworkable. Therefore, several nonbiological tests have been introducedand utilized for measurement of dissolved organic matter. One such testis known as the chemical oxygen demand (COD) test which involves atwo-hour dichrornate reflux period for the sample with concentratedsulfuric acid-dichromate solutions. This test is rou tinely utilized bythe Public Health Service in water quality monitoring. In addition tothe COD test, several automated oxygen demand and analyzers have beenintroduced and are currently being utilized. These automatic oxygendemand analyzers normally utilize the principle of complete combustionof the dissolved organic materials to the final oxidation products ofcarbon dioxide and water, and an analysis of the product gases providesa measure of the oxygen demand.

The method and apparatus of the present invention provides for the rapiddetermination of water quality by means of a voltage scanning techniquewherein a linearly increased DC voltage is applied across a pair ofspaced electrodes and the current, as a function of this voltage, iscontinuously monitored. In accordance with this technique, the dissolvedoxygen level, the relative amount of unstabilized organics present inthe sample, as well as the relative concentration of dissolved chlorideion among other constituents may be determined.

SUMMARY OF THE INVENTION Briefly, in accordance with the presentinvention, a sample transmission or flow chamber is provided and a groupof three electrodes are arranged within the chamber in spacedrelationship, preferably spaced along the direction of sample flowthrough the chamber, and in contact with the sample. The threeelectrodes are an indicating electrode, a reference electrode, and athird, or common, electrode, which is in circuit with the reference andthe indicating electrodes.

In operation, the sample is acidified, and is thereafter passed throughthe flow chamber. A low current, preferably of the order of a fewmilliamps, is passed between the common and reference electrodes, thiscurrent being obtained from a constant current DC source. Underconditions of current flow, hydrogen gas is evolved from one of theelectrodes, with oxygen gas being evolved from the other. The observedreference voltage or reference point is obtained from the electrodeevolving hydrogen, this electrode being polarized and maintaining aconstant voltage since the hydrogen ion concentration is fixed by thesample flow through the cell. The pressure of the hydrogen gas evolvedat the electrode is also constant, this pressure being one atmosphere.

The indicator electrode and the common electrode form the two electrodepolarographic portion of the system. The current read-out is obtained bymeasuring a voltage drop across a standard resistor.

In accordance with this system, the oxygen content of the sample may bedetermined by noting the value of the voltage when the current increasesrapidly in the negative direction. The output curve moves toward morepositive values as the oxygen level of the sample increases. The secondcharacteristic portion of the curve is responsive to the oxidation ofunstabilized organic material present in the sample, this portion of thecurve occurring at an applied voltage of about 1.4 to 1.5 volt, where acurrent peak occurs. A lowering in the amount of unstabilized organicmaterial in the sample is indicated by the decrease in current as afunction of processing time. The third characteristic portion of thecurve occurs in response to chloride ion concentration, this portionoccurring at a point where the current begins to increase rapidly in thepositive direction. This current rise moves toward less positive voltageas the chloride ion concentration increases.

Therefore, it is a primary object of the present invention to provide animproved system for the determination of water quality in aqueoussamples by electrolytic techniques, the

system being adaptable for rapid and continuous determinations of thosecertain parameters which determine the quality of the sample.

It is a further object of the present invention to provide a rapid andcontinuous technique for the determination of certain parameters ofwater quality, including a determination of dissolved oxygen content,the presence of unstabilized organic material in the sample, along withchloride ion concentration.

It is yet a further object of the present invention to provide a rapidand continuous technique for the determination of certain parameters ofwater quality including dissolved oxygen content, presence ofunstabilized organic material, and chloride ion concentration, thetechnique employing a simple electrochemical cell utilizing electricalmeasuring techniques which may be expeditiously accomplished without theneed for expensive or unusual components.

Other and further objects of the present invention will become apparentto those skilled in the art upon a study of the following specification,appended claims and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of theelectrochemical system utilized for measuring water quality, orpollution monitoring;

FIG. 2 is a schematic diagram of the electrochemical cell utilized inthe system of FIG. 1;

FIG. 3 is a typical curve obtained with the apparatus shown in FIGS. 1and 2, the curves showing the relationship between the pollution leveland extent of water processing of a typical sewage plant effluent;

FIG. 4 is a curve illustrating the relationship between pollutioncharacteristics of a typical lake sample and a surface sample collectedfrom a creek flowing into the lake;

FIG. 5 is a schematic diagram of a circuit which may be employed in thesystem of the present invention;

FIG. 6 is a diagram of a system of the present invention which may beutilized for the detection of carbon monoxide; and

FIG. 7 is a curve illustrating typical peak current values obtained as afunction of carbon monoxide content of the gas sample uses the currentin the operation of the apparatus of FIG. 6 and using a scan of 20seconds.

DESCRIPTION OF THE PREFERRED EMBODIMENT In accordance with the preferredembodiment of the present invention, and with particular reference beingdirected to FIG. 1 of the drawings, the block diagram shows thearrangement of the components forming the electrochemical instrument ofthe present invention which is adaptable for use in determination ofwater quality or pollution monitoring. The system includes a flowchamber or cell 10 which is adapted to receive incoming sample materialby means of a suitable conduit arrangement or scheme, as shown, theincoming sample initially being passed through the sample container andpreheating chamber 11 where the temperature of the sample is raised to alevel substantially equal to the testing temperature desired. Uponleaving the pre-heating chamber 11, the material flows through theheater chamber 12 where it is heated to the final testing temperature,and is thereafter passed through the flow chamber or cell 10. Uponleaving the cell 10, the sample is discharged from the conduit system,as indicated. A suitable pumping means is provided for circulating thesample through the system, such as the pump shown at 13, this pump beingeither of the positive displacement type or vane type, it beingunderstood that any suitable pumping source may be employedsuccessfully. In order to avoid pulsations in current flow, it isgenerally desirable to employ a pump system which has a substantiallysteady-state output, which is free of pulsations or fluctuations inpressure or flow, this condition being readily obtainable by proper pumpselection, or by the use of pressure accumulators or the like. Suchsystems are, of course, commercially available.

The sample container and pre-heating chamber 11 is preferably providedwith a resistance heater or the like which is capable of elevating thetemperature of the incoming sample to a level which is just slightlybelow the testing temperature. Upon leaving chamber 11, the sample flowstoward the flow chamber or cell 10, and passes adjacent a second heaterarrangement such as the sample heater 12, which raises the temperatureof the sample to the desired level. Because of the control desired, atemperature controller and heater control are provided for controllingthe operational characteristics of the sample heater arrangement 12.While passing through the flow chamber or cell 10, the sample issubjected to the influence of a reference voltage, the response of thecell to this reference voltage being indicative of certain qualitydetermining parameters of the sample.

In order to better comprehend the function of the cell, attention is nowdirected to FIG. 2, with continued attention being directed to FIG. 1.In FIG. 2, which is a schematic of the flow chamber or cell 10, a groupof electrodes are illustrated including a reference electrode 15 coupledto a reference electrode power supply, and an indicating electrode 16,which is coupled to a voltage scanner. A third, or common, electrode 17is arranged in common and in circuit with both the reference electrodepower supply, as indicated, and the voltage scanner, as indicated. Theseelectrodes are preferably fabricated from platinum sheet, and areoriented parallel to the stream flow so as to prevent or reduce gasbubble and particle build-up on the electrode surfaces. By way ofexample, for a flow chamber or cell having a general cylindricalconfiguration, with an ID. of 5.5 millimeters, the reference electrodes,electrodes 15 and 17, were 1.0 cm X 0.5 cm, while the indicatingelectrode 16 was 0.5 cm X 0.5 cm. All electrodes were 5 mils inthickness.

While electrode spacing is generally not critical, a spacing ofapproximately one inch is desired between the leading edges of theindicating electrode 16 and the common electrode 17, while a spacing of2 inches is desired between the leading edges of the reference andcommon electrodes 15 and 17 respectively. In order to minimizeturbulence, it is normally preferred that there be a reasonable lead-insection provided in the cell, such as is illustrated at 18, this sectionbeing utilized to reduce turbulence, cavitation, or the like in thefluid passing through the cell and undergoing determinations therein.

VOLTAGE SCANNER POWER SUPPLY A source of linearly changing DC voltage isrequired for this portion of the system. For most purposes of waterquality determination, the output should be capable of delivering powerfrom 0 to about 3 volts, this potential being applied across theindicator electrode 16 and the common electrode 17 on the cell 10. Byusing solid-state circuit and a Zener diode stabilized output, there isno need for voltage standardization and the output characteristics arenormally adequate for pollution studies. Power supplies of suchcharacteristics are readily available commercially and are standard inthe art. In the system, such a DC power supply is shown at 20.

A stabilized signal from the DC power supply 20 is applied across ahelipot which is employed to perform the function of limiting thevoltage scan. By adjusting of this resistor, the voltage range scannedis preferably variable from 0 volt to a maximum of about 3.5 volts. Thescan rate is selected by appropriate adjustment of the motor driving thehelipot, this arrangement being incorporated in the block diagram ofFIG. 1 at 21. Motor driven helipots are also readily availablecommercially, and may of course, be appropriately selected for use inconnection with this apparatus.

REFERENCE ELECTRODE POWER SUPPLY The reference electrode power supply isvery simple, and normally consists of two variable resistors which areof high enough resistance to limit the current flow regardless of smallimpedance changes within the cell. Preferably, a 6 volt-lantem batterymay be utilized to supply power for this constant current supply withread-out being obtained on a simple meter. This power supply is shown inthe block diagram of FIG. 1 at 22. Also, see FIG. 5.

AUXILIARY EQUIPMENT As has been previously indicated, two heaters arenormally employed for temperature control. A first heater is providedfor the sample container and pre-heating chamber 11, which, asindicated, may be a simple immersion type resistance heater or the like.This heater must be capable of raising the temperature of the sample,for a flow rate of a minimum 100 cc/min. to a temperature of about 35 C.Final temperature regulation is obtained by means of the heater retainedwithin the sample heater chamber 12, this heater preferably being aresistance heater which is in contact with the sample per se, or theconduit carrying the sample. The electrical input of this resistanceheater is controlled by temperature controller 24 operating throughheater control 25. Temperature controller 24 may be, for convenience, athermistor sensor which operates to control the current flow throughheater control 25. Temperature controllers and heater controls of thistype are, of course, commercially available.

In order to achieve readout of the conditions within the flow chamber orcell 10, a strip chart recorder or electronic ammeter may be utilizedfor the current read-out, this structure being shown in the blockdiagram of FIG. 1 at 26. For proper operation of the recorder, currentselector switch 27 is provided to assist in the operation of theread-out recorder 26.

TYPICAL OPERATION OF THE SYSTEM With continued attention being directedto FIG. 1, it will be seen that the incoming sample is carried throughsuitable conduit means to the sample container and pre-heating chamber11. Prior to its introduction into the chamber 11, a predeterminedquantity of a mineral acid such as sulfuric acid is added to the sampleuntil a final concentration of 0.35 N H 80 is obtained. A control valveat 31 monitors the flow of acid into the sample, with conduit means 32communicating with the conduit carrying the incoming sample, asindicated. The acidified samples are pre-heated to about 35 C. inchamber 11, and are thereafter passed through sample heater chamber 12,where the temperature is increased to about 40 C. The tolerance levelfor the sampling material leaving sample heater 12 is preferably of theorder of about fl.0 C.

With the pump 13 calibrated at a flow of 100 cc/min., the referenceelectrode power supply 22 was turned on and a current flow of 5 ma wasestablished. As is indicated in FIG. 1, only one pass of the samplematerial was utilized.

Upon achieving the stability in temperature an flow rates, the scanningvoltage is applied to the cell across electrodes 16 and 17. The scanningvoltage is selected to range from 0 up to 2.5 volts. The current wasmonitored by voltage measurement (displayed on recorder 26) across astandard resistor, placed into the circuit by means of the currentselectivity switch 27. The time axis of the recorder was also utilizedas a voltage axis for the applied voltage, since a linear scan wasutilized and the voltage axis of the recorder served as the currentdisplay.

RESULTS OBTAINED ON SEWAGE SAlVIPLES The results of the pollutionstudies obtained from sewage samples are illustrated in terms of thedata obtained in the curves of FIG. 3. As is indicated, the relativeconcentrations of dissolved oxygen, the relative amount of unstabilizedorganic material present in the sample, as well as the relativeconcentration of dissolved chloride ion are graphically illustrated. Therelative amounts of dissolved oxygen present in the sample, along withthe relative amounts of the pollutants including unstabilized organicmaterial and dissolved chloride ion can be determined at three pointsalong the current-voltage curves.

The oxygen content can be determined by noting the value of the voltagewhen the current increases rapidly in the negative direction. The curvemoves toward more positive voltages as the oxygen level increases.Noting FIG. 3, this means the oxygen level increases as the sampleapproaches the lake.

The second characteristic part of the curve is at an applied voltage ofabout 1.4 to 1.5 volts where a current peak occurs. This peak is due tothe oxidation of unstabilized organic material in the sample. Thismaterial is continually acted upon by bacteria along the path to thelake until the degradation is nearly complete. The lowering in theamount of unstabilized material is indicated by the increase in currentas a function of processing time (FIG. 3).

The third characteristic part of the curve is the voltage at which thecurrent begins to increase rapidly in the positive direction. Thiscurrent rise moves toward less positive voltages as the chloride ionincreases. In FIG. 3 it is apparent that the chloride ion content isunchanged as the eflluent proceeds through the plant, but the chlorideion concentration is much lower in the lake proper.

Attention is now directed to FIG. 4 of the drawings wherein the curveillustrates the relationship between pollution characteristics of atypical lake surface sample and a surface sample collected from a creekwhich flows into the lake. The dissolved oxygen level is relatively highin the lake, and is relatively low in the creek. The concentration ofunstabilized organics appears high in the creek, and relatively lower inthe lake surface sample. There is no significant difference indicated inchloride ion concentrations.

It will be appreciated that the system of the present invention may beemployed on shipboard. As such, it may be utilized as a generalpollution detector for determining the source of introduction ofpollutants into a body of water, if desired, or for other purposes.Since the instrument provides a test time for only several minutes orless, a shipboard installation could be utilized to map a body of waterfor the presence of several pollutants, which may occur in the waterbody. The presence of unstabilized organic material and chloride ionindicate waste discharge into a body of water, with organic wasteoriginating from human excreta and household wastes, while chloride ionnormally originates from water regeneration systems, household wastesand water run-off. Industrial pollutants may also be determined in thisfashion.

Attention is now directed to FIG. 5 of the drawings which illustrates atypical circuit useful in connection with the present invention. Thesystem includes a conventional l20-v0lt power source, as indicated,which is connected across the mains 40 and 41. A ganged switch 42 isutilized to turn the system on, and indicator light 43 is utilized torepresent the immediate situation. The l20-volt supply source suppliespower to drive the pump member 13, which corresponds to the structureshown in FIG. 1, as well as the potentiometer driver motor 45.

With attention now being directed to the lower portion of the schematicof FIG. 5, this arrangement is energized upon closing of the contacts 47of switch 42, thus closing the circuit to the electrochemical componentsof thesystem. Reference electrode 15 is coupled to the positive pole ofbattery 48 through series resistor 49, while common electrode 17 iscoupled to the negative pole of battery 48 along bus 50. Bus 50 iscoupled to one terminal of the potentiometer winding 51 by way ofresistor 52, with the wiper 53 of the potentiometer being coupled toindicator electrode 16 through meter 54 and switch element 55. Arecorder jack is provided at 56 for the purpose of receiving the outputfrom indicator electrode 16. The other terminal of potentiometer winding51 is coupled to the positive pole of battery 48 through resistor 58,thus completing the circuit.

The following table is provided to indicate typical circuit values:

Table I Battery 48 Three 1.35 volt mercury cells Resistor 49 390 ohms 68ohms Resistor 52 Potentiometer 51 100 ohms, 360 mechanical rotationResistor 58 150 ohms Meter 54 I microamp meter Motor 45 RPM synchronousmotor ELECTROCHEMICAL CARBON MONOXIDE DETECTOR Attention is now directedto FIG. 6 of the drawings wherein a system for the detection anddetermination of carbon monoxide utilizing the principles of the presentinvention, is schematically illustrated. In this system, a sample of agas from a source which may contain carbon monoxide is coupled toconduit 60 communicating with compressor 61. The output of compressor 61delivers the sample through conduit 62 into test vessel 63, dischargingthe gas into the solution contained therein through the port 64. Thesolution contained within vessel 63, shown at 65, is preferably 0.5 N H50 Vessel 63 is provided with three spaced electrodes, these electrodescorresponding to reference electrodes and 17, and indicating electrode16. An electrical input is provided to these electrodes from the scaninput arrangement shown at 67, this scan input being, of course, basedupon the circuit shown in FIG. 5. As is illustrated in FIG. 6, a powersupply is provided as at 68, with a meter read-out device shown at 69,along with a recorder read-out device as shown at 70. These componentsare also conventional in the art and there is accordingly no unusualrequirement for these features of the system.

In operation the conduit means 60 is initially coupled to the source ofcarbon monoxide, such as, for example, the exhaust pipe of an internalcombustion engine, and this input is then pumped by means of compressorelement 61 into the sulfuric acid solution 65 retained in vessel 63. Inthis arrangement, therefore, the reference electrode 15 functions as avoltage reference, and electrodes 16 and 17 function to indicate thepresence of electro-active material in the solution. For carbon monoxideoxidation reactions, the voltage scan extends from 0.7 volt to 1.5 voltover a period of between 20 and 60 seconds. Under these conditions,electrolysis of the solution occurs and the electrolysis current islimited by the diffusion or adsorption of the electroactive material.This electrolysis current, therefore, may be used to monitor theconcentration of the electro-active carbon monoxide present in thesolution. The current-voltage curves are reproducible since theelectrode surface is continually renewed with each scan. The observeddifference in current levels achieved with and without theelectro-active material present is proportional to the bulkconcentration of the electro-active material in the solution. In thiscase, carbon monoxide is the electro-active material. It will beappreciated that the operational characteristics of the carbon monoxidedetector are not significantly different from those utilized in thedetection of other electro-active materials in solutions, such asoxygen, organic products, and the like.

With attention being directed to FIG. 7, it will be appreciated that theoutput curve achieved represents the percentage of carbon monoxidepresent in the gaseous mixture forming the source of carbon monoxide.

The response time of this apparatus when employed as a carbon monoxidedetector, is a function of two related properties, the first being theelectrode response to the carbon monoxide dissolved in the electrolyte,the second being the equilibration time required for the dissolution ofgaseous carbon monoxide. Since electrode response time is negligible,the total limiting feature is the equilibration time. In an electrolytevolume of about 50 cc., carbon monoxide concentration ranging from 0percent up to about 3 percent may be detected in substantially 1 minute,and if the volume of electrolyte is diminished, time can be reducedaccordingly.

The apparatus responds to carbon monoxide along a reasonably linearcurve from 0 up to about 1.5 percent of can bon monoxide, this curvethen tailing to where only modest sensitivity was observed above about 7percent. The curve is reasonably linear and highly reproducible in theranges of from between about 0.25 percent and 5 percent.

THEORETICAL CONSIDERATIONS It will be appreciated that the electrodearea is selected so as to be sufficiently small to limit current flowthrough the cell. Since very little electro-active material is consumedin the electrochemical reaction, a much shorter test time is required.The potential which is applied to the cell is sufficiently low so as tobe below the potential required for the evolution of oxygen. Thus, onlyreactions involving the oxidation of organic and selected inorganicsubstances occur. The voltage-current relationship detemrined prior tothe evolution of oxygen is normally primarily a function of theconcentration of the electro-active material present in the sample.

The magnitude of the observed current for a given concentration ofelectro-active material depends upon two factors. The first is thedifiusion rate of reactant to the electrode, and if this rate is small,only a few electrons can be transferred in a given time period. Thisrate is dependent primarily upon temperature, rate of sample motion,molecular size, electrode size, solvent effects, and most importantly,the bulk concentration of reactant.

The second factor involves the reaction rate at the electrode surfaces.Once the reactant arrives at the electrode surface, a number of stepsoccur. Generally, some form of the reactant is absorbed into thesurface, electron transfer occurs, product desorption processes follow,and with product removal, a continual supply of reactant reaches theelectrode. Each stage can be described by an appropriate rate expressionwhich, taken in total, indicates the electrochemical reaction rate.

In a given sample, before the electro-oxidation of electroactivematerials begins, the current is practically identical to the chargingcurrent, that is, the i-E relationships are due primarily to the puresupporting electrolyte. This portion of the curve is followed by a rangeof potential were a steep increase in current is observed. This steepincrease is followed by a narrow range of potential where the currentremains practically constant and parallel to the charging current. Thisleveling off is characteristic of reversable inorganic systems, and iscalled the limiting current or wave height for the system. For organicsystems, however, the electrode reactions which occur result in alessening in diffusion control which is manifested by a diminishingcurrent. It is likely that the adsorption rate of electrode blockingby-product slows the reaction, however, the current is observed to dropbecause of the depletion of material available at the electrode. Thisobserved as a peak on the i-E curve.

Generally, voltametric procedures are applied to single species existingin solution. In such cases, diffusion currents can be assigned. Thediffusion current is proportional to the bulk concentration according tothe well-known Ilkovic Equation. The peak current that is observed insewage runs is found to be a proportional to the bulk concentration ofunstabilized organic materials present.

For most determination operations, a scan rate in the area of about 0.3to 3 volts/min. was employed. Normally, a scan rate of about 0.3volt/min. is preferred.

The preferred electrolyte for use in the samples undergoingdetermination is sulfuric acid, and concentrations ranging from about0.25 N up to about 1.0 N may be successfully utilized. While othermineral acids may be employed, it is appreciated that sulfuric acid ispreferred as a supporting electrolyte inasmuch as sulfates are normallyelectrically inactive.

The results obtained with the apparatus of the present invention comparefavorably with BOD determinations.

We claim:

I. In a system for the determination of the electro-activity ofingredients present in aqueous samples:

a. cell means for receiving an aqueous sample containing a supportingelectrolyte and electro-active components undergoing determination;

b. a plurality of spaced electrodes including a reference electrode, anindicating electrode, and a common electrode, each electrode beingdisposed within said cell and in contact with said sample;

c. first circuit means coupled to said reference electrode and commonelectrode for impressing a unipolar electrical signal thereacross of amagnitude sufficient for aqueous electrolysis, and adapted to deliver acurrent of constant magnitude;

. second circuit means coupling said common electrode to said indicatingelectrode;

e. signal generating means in said second circuit means for applying aunipolar scanning signal across said indicating electrode and saidcommon electrode, said scanning signal having a programmed potentialmagnitude with predetermined increases and decreases with time; and

f. electrical current read-out means in said second circuit means and inseries with said common electrode and said indicating electrode fordetermining the current flow through said second circuit means inresponse to said scanning signal.

2. The system as defined in claim 1 being particularly characterized inthat means are provided for maintaining said aqueous sample at asubstantially constant temperature.

3. The system as defined in claim 1 being particularly characterized inthat heating means are provided for maintaining said sample at a certainpredetermined temperature.

4. The system as defined in claim 1 being particularly characterized inthat means are provided for passing a fluid sample undergoingdetermination through said cell.

5. The system as defined in claim 1 being particularly characterized inthat means are provided for moving a gaseous fluid sample containing anelectro-active component through said cell.

6. The system as defined in claim 1 being particularly characterized inthat said indicating electrode and common electrode are spaced apart bya predetermined distance and wherein said reference electrode and saidcommon electrode are spaced apart by a distance which is greater thansaid predetermined distance.

7. In a system for the substantially continuous determination ofelectro-activity of ingredients present in aqueous samples:

a. cell means for receiving a flow of an aqueous sample with means fordelivering and discharging the flow of said sample to and from saidcell;

b. means delivering a predetermined quantity of a water solubleelectrolyte into said sample for maintaining said electrolyte in saidsample at a predetermined concentration level;

c. a plurality of spaced electrodes including a reference electrode, anindicating electrode, and a common electrode, each electrode beingdisposed within said cell and in contact with said sample;

d. heating means for maintaining said sample at a certain predeterminedtemperature level within said cell;

e. first circuit means coupled to said reference electrode and commonelectrode for impressing a unipolar electrical signal thereacross of amagnitude sufficient for aqueous electrolysis, and adapted to deliver acurrent of constant magnitude;

. second circuit means coupling said common electrode to said indicatingelectrode;

g. signal generating means in said second circuit means for applying aunipolar scanning signal across said indicating electrode and saidcommon electrode, said scanning signal having a programmed potentialmagnitude with predetermined increases and decreases with time; and

h. electrical current read-out means in said second circuit means and inseries with said common electrode and said indicating electrode fordetermining the current flow through said second circuit means inresponse to said scanning signal.

8. The system as defined in claim 7 being particularly characterized inthat said spaced electrodes are arranged along a predetermined line.

9. The system as defined in clarm 8 being partrcularly characterized inthat said indicating and common electrode are spaced apart by apredetermined distance and wherein said reference electrode and saidcommon electrode are spaced apart by a distance which is greater thansaid predetermined distance.

10. The system as defined in claim 7 being particularly characterized inthat said signal generating means in said second circuit means iscoupled to a source of electrical energy and is adapted to apply aunipolar scanning signal of a magnitude less than that required foraqueous electrolysis.

11. The system as defined in claim 7 being particularly characterized inthat each of said electrodes consists of a precious metal.

12. The system as defined in claim 11 wherein each of said electrodesconsists essentially of platinum.

13. The system as defined in claim 9 being particularly characterized inthat said distance between said reference electrode and said commonelectrode is twice said predetermined distance.

14. The system as defined in claim 7 being particularly characterized inthat said scanning signal has a peak amplitude less than about 2.5volts.

15. The system as defined in claim 7 being a particularly characterizedin that said indicating electrode and common electrode are spaced apartby a certain predetermined distance, and wherein said referenceelectrode and said common electrode are spaced apart by a distance whichis greater than said predetermined distance.

1. In a system for the determination of the electro-activity ofingredients present in aqueous samples: a. cell means for receiving anaqueous sample containing a supporting electrolyte and electro-activecomponents undergoing determination; b. a plurality of spaced electrodesincluding a reference electrode, an indicating electrode, and a commonelectrode, each electrode being disposed within said cell and in contactwith said sample; c. first circuit means coupled to said referenceelectrode and common electrode for impressing a unipolar electricalsignal thereacross of a magnitude sufficient for aqueous electrolysis,and adapted to deliver a current of constant magnitude; d. secondcircuit means coupling said common electrode to said indicatingelectrode; e. signal generating means in said second circuit means forapplying a unipolar scanning signal across said indicating electrode andsaid common electrode, said scanning signal having a programmedpotential magnitude with predetermined increases and decreases withtime; and f. electrical current read-out means in said second circuitmeans and in series with said common electrode and said indicatingelectrode for detErmining the current flow through said second circuitmeans in response to said scanning signal.
 2. The system as defined inclaim 1 being particularly characterized in that means are provided formaintaining said aqueous sample at a substantially constant temperature.3. The system as defined in claim 1 being particularly characterized inthat heating means are provided for maintaining said sample at a certainpredetermined temperature.
 4. The system as defined in claim 1 beingparticularly characterized in that means are provided for passing afluid sample undergoing determination through said cell.
 5. The systemas defined in claim 1 being particularly characterized in that means areprovided for moving a gaseous fluid sample containing an electro-activecomponent through said cell.
 6. The system as defined in claim 1 beingparticularly characterized in that said indicating electrode and commonelectrode are spaced apart by a predetermined distance and wherein saidreference electrode and said common electrode are spaced apart by adistance which is greater than said predetermined distance.
 7. In asystem for the substantially continuous determination ofelectro-activity of ingredients present in aqueous samples: a. cellmeans for receiving a flow of an aqueous sample with means fordelivering and discharging the flow of said sample to and from saidcell; b. means delivering a predetermined quantity of a water solubleelectrolyte into said sample for maintaining said electrolyte in saidsample at a predetermined concentration level; c. a plurality of spacedelectrodes including a reference electrode, an indicating electrode, anda common electrode, each electrode being disposed within said cell andin contact with said sample; d. heating means for maintaining saidsample at a certain predetermined temperature level within said cell; e.first circuit means coupled to said reference electrode and commonelectrode for impressing a unipolar electrical signal thereacross of amagnitude sufficient for aqueous electrolysis, and adapted to deliver acurrent of constant magnitude; f. second circuit means coupling saidcommon electrode to said indicating electrode; g. signal generatingmeans in said second circuit means for applying a unipolar scanningsignal across said indicating electrode and said common electrode, saidscanning signal having a programmed potential magnitude withpredetermined increases and decreases with time; and h. electricalcurrent read-out means in said second circuit means and in series withsaid common electrode and said indicating electrode for determining thecurrent flow through said second circuit means in response to saidscanning signal.
 8. The system as defined in claim 7 being particularlycharacterized in that said spaced electrodes are arranged along apredetermined line.
 9. The system as defined in claim 8 beingparticularly characterized in that said indicating and common electrodeare spaced apart by a predetermined distance and wherein said referenceelectrode and said common electrode are spaced apart by a distance whichis greater than said predetermined distance.
 10. The system as definedin claim 7 being particularly characterized in that said signalgenerating means in said second circuit means is coupled to a source ofelectrical energy and is adapted to apply a unipolar scanning signal ofa magnitude less than that required for aqueous electrolysis.
 11. Thesystem as defined in claim 7 being particularly characterized in thateach of said electrodes consists of a precious metal.
 12. The system asdefined in claim 11 wherein each of said electrodes consists essentiallyof platinum.
 13. The system as defined in claim 9 being particularlycharacterized in that said distance between said reference electrode andsaid common electrode is twice said predetermined distance.
 14. Thesystem as defined in claim 7 being particularly characterized in thatsaid sCanning signal has a peak amplitude less than about 2.5 volts. 15.The system as defined in claim 7 being a particularly characterized inthat said indicating electrode and common electrode are spaced apart bya certain predetermined distance, and wherein said reference electrodeand said common electrode are spaced apart by a distance which isgreater than said predetermined distance.